The present invention relates to a radio transmission output control circuit and, more particularly, to a radio transmission output control circuit for controlling the transmission outputs of consecutive time slots in TDMA (Time Division Multiple Access) communication.
A radio transmission output control circuit for the above application is disclosed in, for example, FIG. 1 of Japanese Utility Model Laid-Open Publication No. 64-51330. The control circuit of this document includes a ROM (Read Only Memory) 3 storing transmission output switching signals each being assigned to a particular channel. When a certain channel is selected on a channel set switch 1, one of the switching signals matching the channel is read out of the ROM 3 under the control of a microcomputer 2. The switching signal is applied to an automatic power control circuit 5. The power control circuit 5 automatically controls, based on the switching signal, the transmission output of a power amplifier 6 connected to a transmitter section 4.
Digital communication is predominant in today's communication environments. In addition, modulation for digital communication is, in most cases, implemented by a .pi./4 shift QPSK (Quadrature Phase-Shift Keying) system. A digital cordless telephone is one of TDMA communication apparatuses using the .pi./4 shift QPSK system, and is a four-slot multiplex communication TDMA-TDD. However, the problem with this kind of modulation system is that the modulated signal contains an AM (Amplitude Modulation) component. Hence, if the level of the transmission output is varied in the same time slot, a data error occurs. It is, therefore, necessary to control, within a guard time intervening between consecutive time slots, the transmission output and stabilize its level. The guard time available with the digital cordless telephone is as short as about 30 .mu.s. That is, the control over the transmission output and the stabilization of its level must be completed within 30 .mu.s. However, high-speed processing is not achievable without complicating the structure of the radio transmission output control circuit. A complicate structure directly translates into an increase in size and power consumption. This kind of control circuit is, therefore, not applicable to a digital cordless telephone or similar portable apparatus.